The present invention relates in general to radio frequency (RF) communication systems, and is particularly directed to an RF power amplifier distortion detection and correction mechanism, that employs a pair of frequency swept input and output receivers to locate and isolate the RF carrier component in each of predistortion, carrier cancellation and feed-forward cancellation loops of an RF power amplifier circuit, so that distortion energy associated therewith may be detected. Once detected, distortion energy may be removed by controlling the parameters of a pre-distortion unit and a gain/phase adjustment unit installed in the predistortion path to the main amplifier, and a gain/phase adjustment unit installed in the feed-forward cancellation path.
The specifications and regulations of the Federal Communications Commission (FCC) currently mandate that communication service providers comply with very strict bandwidth constraints, including the requirement that the amount of energy spillover outside a licensed channel or band of interest, be sharply attenuated (e.g., on the order of 50 dB). Although such limitations may be overcome for traditional forms of modulation, such as FM, they are difficult to achieve using more contemporary, digitally based modulation formats, such as M-ary modulation.
Attenuating sidebands sufficiently to meet industry or regulatory-based standards using such modulation techniques requires very linear signal processing systems and components. Although relatively linear components can be obtained at a reasonable cost for the relatively low operating frequencies (baseband) of telephone networks, linearizing components such as power amplifiers at RF frequencies can be prohibitively expensive.
A fundamental difficulty in linearizing an RF power amplifier is the fact that it is an inherently non-linear device, and generates unwanted intermodulation distortion products (IMDs). IMDs manifest themselves as spurious signals in the amplified RF output signal, separate and distinct from the RF input signal. A further manifestation of IMD is spectral regrowth or spreading of a compact spectrum into spectral regions that were not occupied by the RF input signal. This distortion causes the phase-amplitude of the amplified output signal to depart from the phase-amplitude of the input signal, and may be considered as an incidental (and undesired) amplifier-sourced modulation of the RF input signal.
A straightforward way to implement a linear RF power amplifier is to build it as a large, high power device, but operate the amplifier at a only a low power level (namely, at a small percentage of its rated output power), where the RF amplifier""s transfer function is relatively linear. An obvious drawback to this approach is the overkill penaltyxe2x80x94a costly and large sized RF device. Other prior art techniques which overcome this penalty include feedback correction techniques, feedforward correction, and pre-distortion correction.
Feedback correction techniques include polar envelope correction (such as described in U.S. Pat. No. 5,742,201), and Cartesian feedback, where the distortion component at the output of the RF amplifier is used to directly modulate the input signal to the amplifier in real time. Feedback techniques possess the advantage of self-convergence, as do negative feedback techniques in other fields of design. However, systems which employ negative feedback remain stable over a limited bandwidth, which prevents their application in wide-bandwidth environments, such as multi-carrier or W-CDMA. Feedforward and predistortion correction, however, are not limited in this regard. In the feedforward approach, error (distortion) present in the RF amplifier""s output signal is extracted, amplified to the proper level, and then reinjected with equal amplitude but opposite phase into the output path of the amplifier, so that (ideally) the RF amplifier""s distortion is effectively canceled.
When predistortion correction is used, a signal is modulated onto the RF input signal path upstream of the RF amplifier. The characteristic of an ideal predistortion signal is the inverse of the distortion expected at the output of the high power RF amplifier, so that when subjected to the distorting transfer function of the RF amplifier, it effectively cancels the distortion behavior.
Either predistortion or feedforward may be made adaptive by extracting an error signal component at the output of the RF amplifier and then adjusting the control signal(s), in accordance with the extracted error behavior of the RF amplifier, so as to effectively continuously minimize distortion in the amplifier""s output.
One conventional mechanism for extracting the error signal component involves injecting a pilot (tone) signal into the signal flow path through the amplifier and measure the amplifier""s response. A fundamental drawback of using a pilot tone is the need for dedicated pilot generation circuitry and the difficulty of extracting and measuring the pilot tone within the signal bandwidth of the amplifier. Other approaches employ a high intercept receiver to detect low level distortion in the presence of high power carriers, which adds substantial complexity and cost.
In accordance with the present invention, RF power amplifier distortion is accurately measured, even in the presence of multi-frequency input signals, by using a controllably swept local oscillator to tune respective RF input and output receivers, the energy content of which is monitored for distortion correction purposes. Power detected by the input receiver, which is coupled to monitor the RF input to a main RF power amplifier, is compared with a carrier power threshold to determine the presence of carrier at the main amplifier""s input.
A predistortion RF signal flow path from the RF input port to the main amplifier includes a gain/phase adjustment circuit controlled by amplitude and phase adjustment signals from a performance monitoring and parameter updating digital signal processor (DSP). The DSP executes one or more error minimization algorithms (e.g., power or least mean squared) for adjusting parameters imparted to respective signal transport loops associated with the amplifier, as will be described. The signal transport path further includes a predistortion unit which is operative to dynamically adjust the amplitude and phase of the RF input signal to main RF amplifier and may contain a vector modulator driven by a complex polynomial work function. The predistortion unit is coupled to receive weighting coefficients from the DSP.
A portion of the output of the main RF amplifier representative of the amplified original RF input signal and any IMDs introduced by the RF amplifier is coupled to a carrier cancellation combiner, which is operative to cancel, from the output of the RF amplifier, a delayed (time aligned) RF carrier component supplied by an RF error signal flow path from the carrier cancellation combiner, so as to provide an RF error signal representative of IMDs. The RF input signal flow path is further coupled to the RF carrier cancellation combiner. The RF error signal produced by the RF cancellation combiner is coupled to a DSP-controlled gain/phase adjustment circuit in the signal flow path to a feed-forward RF error amplifier, whose output is reinjected into the output path of the main RF amplifier.
In order to monitor the RF input signal for the presence of carrier energy, the signal flow path to the carrier cancellation combiner is coupled to a mixer within a controllably swept input receiver. A controllably swept output receiver includes a mixer coupled to receive the output of a DSP-controlled multiplexer or switch. The switch has a first input port coupled to the output of the carrier cancellation combiner, and a second input port coupled to the output path of the main RF amplifier. Each of the input and output receivers shares a common local oscillator (LO) which is controllably swept in frequency by a digital sweep-control signal applied by the DSP.
The IF output of the input receiver""s mixer is filtered by a wider band bandpass filter and coupled to a carrier power detector. The carrier power detector is coupled to a threshold detector, the output of which is coupled to a carrier detect input of the DSP. When the output of carrier power detector does not exceed a prescribed threshold associated with an RF carrier signal, the output of the threshold detector is at a first logic state. However, if the carrier power detector detects carrier power in excess of the prescribed threshold, the output of the threshold detector changes to a second logic state. This change in state of the output of the threshold detector is used by the DSP to control the operation of the switch and thereby the signal path to the output receiver.
The output receiver""s mixer has its IF output coupled through a narrower band bandpass filter and an IF buffer amplifier to a distortion power detector. The distortion power detector is coupled through a lowpass filter, digitized and coupled to the DSP. The digitized output of the distortion power detector is integrated and processed by the DSP using one or more error minimization algorithms for controlling the components in the predistortion and feed-forward loops.
During carrier cancellation and feedforward cancellation mode, whenever the power detected by the swept input receiver exceeds the power reference, the signal path through the swept output receiver is multiplexed through the switch to a coupler sampling output of the carrier cancellation combiner. During the sweep of the input and output receivers, the processor stores a digital representation of energy detected by the output receiver into one of two energy accumulation bins. The first bin is a measure of the uncancelled carrier energy at the output of the carrier cancellation combiner. The second bin is a measure of the uncancelled IMD energy at the output of the feedforward amplifier. The two loops may be controlled by minimizing the energy accumulated in their respective bins.
When operating in predistortion mode, the switch is controlled so that the output receiver monitors only the carrier cancellation combiner throughout the sweep. Whenever carrier power is detected by the input receiver, as the input and output receivers are swept, the energy measured by the output receiver will correspond to residual carrier energy at the output of the carrier cancellation combiner and is accumulated in the first bin. When no carrier power is detected by the input receiver as the input and output receivers are swept, the energy measured by the output receiver corresponds to residual distortion energy from the predistorted main amplifier and is accumulated in the second bin. This allows predistortion to be adaptively controlled by minimizing the energy accumulated in the second bin, during a sweep where the output receiver monitors only the carrier cancellation combiner.